The present invention relates to altimeters of the radar type and particularly to altimeters using closed loop tracking circuits. The present invention further relates to means for improving the tracking circuit of a radar altimeter. A typical tracking circuit will utilize a reference signal source which is adapted to be compared with a selected portion of a received video signal. Upon predetermined variations occurring between the two signals, a change in output will cause a change in the indicated altitude and an adjustment made in the tracking loop so that the two signals cancel. If there should be a loss of video signal, an altitude change condition will be indicated and the altimeter circuitry will seek to establish a new altitude indication.
Radar altimeters have been widely used in aircraft for purposes of measuring the altitude of the craft above the terrain immediately below the craft. Under certain operating conditions, particularly conditions such as may be encountered when a helicopter is hovering at low altitudes over certain types of signal absorbing terrain, a return or echo pulse may fluctuate in amplitude and even fade out so that the altimeter cannot carry out its normal terrain tracking or following operation. A typical terrain condition which is likely to create momentary signal fades is one where the surface is flat and sandy, or grassy. Typically, the signal fade condition will be only momentary but the loss of several return or echo pulses will cause the output of the altimeter to go through wide swings so as to render impossible the accurate determination of the altitude.
Representative radar altimeters heretofore known will be found in the following identified references: U.S. Pat. Nos. 3,093,807 L. S. Crane, et al June 11, 1963, 3,249,935 R. J. Follen May 3, 1966, 2,928,085 A. J. Katz Mar. 8, 1960, 3,231,889 W. L. Leyde, et al Jan. 25, 1966, 3,242,488 W. L. Leyde, et al Mar. 22, 1966, 3,251,058 J. A. Sutcliffe May 10, 1966, 3,243,803 B. H. Thue Mar. 29, 1966, 3,344,423 B. H. Thue Sept. 26, 1967, 3,752,582 Troll, et al Aug. 14, 1973.